Method of manufacturing an optical device with a groove accurately formed

ABSTRACT

A first layer (2) and a second layer (3) are formed on a substrate (1). The first layer is made of a resist against a groove-sculpturing etchant used in etching the substrate while the second layer is made of an anti-corrosive material against dry etching. The second layer is at first patterned into a patterned second layer (3a) in the form of a groove-sculpturing mask pattern (8). With the patterned second layer used as a mask, the first layer is etched and patterned into a patterned first layer (2a) in the form of the above-mentioned mask pattern. With the patterned first layer used as a mask, the substrate is etched to form a groove (9).

BACKGROUND OF THE INVENTION:

This invention relates to an optical device and a method ofmanufacturing the optical device.

In order to manufacture an optical device comprising a combination of anoptical waveguide of a fiber mount type and optical and electricalcomponents, various methods have been proposed and put into practicaluse.

For example, Japanese Unexamined Patent Publication (A2) No. H06-347665(347665/1994) discloses a method of manufacturing an optical devicecomprising an optical waveguide formed on a substrate on which opticalcomponents and electronic devices are mounted. According to this method,a mask pattern for use in sculpturing a V-shaped groove or a groove Intide substrate and an electric wiring pattern are formed on thesubstrate (typically, silicon) by the use of a metal material, such asAu, Al, W, and WSi, which is resistant to an anisotropic etchant andelectrically conductive. On the substrate with the mask and is theelectric wiring patterns, the optical waveguide is formed by the use ofa silica glass material. In order to reduce the production cost, theabove-mentioned publication teaches to eliminate a process carried outin presence of step configurations so that high-efficiency opticalcoupling is achieved between the optical components and the opticalwaveguide and that mass production on a substrate scale is enabled inall manufacturing steps, including the formation of electrode pads andthe electric wiring pattern for the electronic devices. To this end,positioning marks for positioning the optical components such as anoptical fiber and an optical semiconductor device, alignment marks foraligning an optical axis of the optical waveguide. the mask pattern, theelectric wiring pattern for the optical components and the electronicdevices, and the electrode pads are formed on the substrate prior to theformation of the optical waveguide.

As will later be described with reference to the drawings the opticalwaveguide is partially etched by wet etching or dry etching to define anend plane thereof and to partially expose a substrate surface.Thereafter, the substrate is subjected to anisotropic etching with themask pattern used as an anisotropic etching mask to form the groove inthe substrate. The groove serves to mount the optical fiber.

In the above-mentioned prior art technique, the mask pattern and theelectric wiring pattern are formed before the step configurations areproduced by the formation of the optical waveguides. In other words, themask pattern and the electric wiring pattern are formed on a flatsurface of the substrate. This allows mass production of the opticaldevice on a wafer scale by the use of a photolithography process.

However, the above-mentioned prior art technique has followingdisadvantages.

Specifically, the anisotropic etching mask formed of the above-mentionedmetal material may often be altered in characteristic under theinfluence of the process of forming the optical waveguides over the maskpattern. This is because the heat produced in this process as well asdopant and moisture present in the optical waveguides are inevitablydiffused into the mask pattern.

Due to such alteration in characteristic, the anisotroplc etching maskloses its resist characteristic against an etchant and no longer servesas a mask. In this situation, the groove of a desired configuration isdifficult to obtain.

As a consequence, it is impossible to accurately and reliably mount theoptical fiber in the groove.

SUMMARY OF THE INVENTION:

It is therefore an object of this invention to provide a method ofmanufacturing an optical device, which is capable of accurately andreliably forming a groove in a substrate without being Influenced by aprocess of forming an optical waveguide on the substrate.

It is another object of this invention to provide a method ofmanufacturing an optical device, which enables an optical fiber to beaccurately and reliably mounted by the use of a groove formed in asubstrate.

It Is still another object of this invention to provide a method ofmanufacturing an optical device, which enables a principal substrate anda fiber guide substrate to be accurately and reliably coupled with eachother by the use of a groove formed in a principal substrate.

It is yet another object of this invention to provide a method ofmanufacturing an optical device having a mounting structure which allowsan optical fiber to be coupled with an optical waveguide withoutrequiring any special adjustment.

Other objects of this invention will become clear as the descriptionproceeds.

According to an aspect of this invention, there is provided a method ofmanufacturing an optical device having a groove. The method comprisesthe steps of forming on a substrate a first layer of a first resistagainst a groove-sculpturing etchant, forming on the first layer asecond layer of a second resist against dry etching, patterning thesecond layer into a patterned second layer having a form relating to thegroove, carrying out dry etching with the patterned second layer used asa mask to pattern the first layer into a patterned first layer, andetching the substrate with the patterned first layer used as a mask toform the groove in the substrate.

According to another aspect of this invention, there is provided anoptical device comprising an optical waveguide formed on a substrate tobe optically coupled with an optical fiber mounted on the substrate. Theoptical device comprises a first layer formed on the substrate andcomprising a resistant against an etchant used in etching of thesubstrate, a second layer formed on the first layer and comprising aresist against dry etching, the first and the second layers beingpatterned into patterned first and patterned second layers in the formof a groove-sculpturing mask pattern used in etching the substrate, anda groove formed on the substrate by etching the substrate with thepatterned first layer used as a mask. The optical waveguide formed onthe substrate is optically coupled with the optical fiber mounted in thegroove formed on the substrate in exact alignment with an optical axisof the optical fiber.

Herein, description will be made as regards a principle of thisinvention. The formation of a waveguide end plane and the exposure of anetch surface of the substrate are simultaneously carried out by dryetching. In this process, the patterned second layer having corrosionresistance to dry etching and patterned in the farm of thegroove-sculpturing mask pattern serves as the mask in dry etching thefirst layer under the patterned second layer. Thus, the first layer ispatterned into the patterned first layer also in the form of thegroove-sculpturing mask pattern. The patterned first layer is made ofthe resist against the groove sculpturing etchant and serves as the maskin etching the substrate to form the groove.

Accordingly, even if the second layer is formed of the materialinherently having no corrosion resistance to the groove-sculpturingetchant or a material losing the corrosion resistance to thegroove-sculpturing etchant under the influence of the process of formingthe optical waveguide on the second layer, the groove-sculpturing maskpattern is exactly reproduced as the patterned first layer without anydamage.

It is therefore possible to accurately and reliably form thegroove-sculpturing mask pattern without being influenced by the processof forming the optical waveguide.

BRIEF DESCRIPTION OF THE DRAWING:

FIG. 1 is a view for describing a conventional process of forming agroove-sculpturing mask pattern;

FIGS. 2A through 2E are views for describing a method of manufacturingan optical device according to one embodiment of this invention;

FIG. 3 is a view similar to FIG. 2A, in which a groove-sculpturing maskpattern and an electric wiring pattern are simultaneously formed;

FIG. 4 is a view similar to FIG. 2D, in which a part of dry etching isreplaced by wet etching; and

FIG. 5 is a view corresponding to FIG. 2E, in which a cut groove isformed at a boundary region between a V-shaped groove and an opticalwaveguide.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

In order to facilitate an understanding of this invention, aconventional method of manufacturing an optical device will be describedin conjunction with FIG. 1. As shown in the figure, a mask pattern 12 isformed on a silicon substrate 1. On the silicon substrate 1 with themask pattern 12, an optical waveguide layer 4 is formed. Thereafter, wetetching or dry etching is carried out with a mask film 5 used as a maskto define a waveguide end plane 6 with a waveguide core end 6a and to asubstrate surface 7a.

Thereafter, anisotropic etching is carried out with the mask pattern 12used as a mask to form a V-shaped groove or a groove in the substrate 1.The groove serves to mount an optical fiber.

Now, description will be made in detail as regards this invention withreference to the drawings.

Referring to FIGS. 2A through 2E, a method of manufacturing an opticaldevice according to one embodiment of this invention will be described.At first referring to FIG. 2A, a silicon substrate 1 is prepared. Afirst layer 2 is formed on the silicon substrate 1. The first layer 2 isformed of a material having corrosion resistance to a groove-sculpturingetchant which is used in etching the substrate 1 in a later stage. Forexample, the first layer 2 is formed of a silica glass materialcontaining silicon dioxide (SiO₂) as a main component. In the opticaldevice, the first layer 2 comprises a silicon dioxide film.

The silicon dioxide film as the first layer 2 can be formed in variousmanners such as CVD (Chemical Vapor Deposition), sputtering, and thermaloxidation. Among others, the silicon dioxide film produced by thermaloxidation is very excellent in corrosion resistance to thegroove-sculpturing etchant and, even with a reduced thickness, fullyserves as a mask in etching the silicon substrate 1 in the later stage.For example, such silicon dioxide film is formed by annealing thesilicon substrate 1 in a steam (H₂ O) atmosphere.

It is noted here that, immediately before etching the silicon substrate1 in the later stage, the silicon dioxide film as the first layer 2 isetched and patterned In the form of a groove-sculpturing mask pattern.Taking this into consideration, the silicon dioxide film is preferablyreduced In thickness. With a reduced thickness, side etching of thesilicon dioxide film is suppressed so that the accuracy in patternformation is improved.

For the reason described above, the silicon dioxide film obtained bythermal oxidation is preferably used as the first layer 2 rather thanthat formed by CVD or sputtering.

In case where the silicon dioxide film obtained by thermal oxidation isused as the first layer 2, the thickness of the silicon dioxide film istypically selected within a range approximately between 100 nanometersand one micrometer. This range is determined with reference to aselection ratio between silicon dioxide and silicon in etching thesilicon substrate 1 by the use of a KOH solution as thegroove-sculpturing etchant.

The first layer 2 may be formed of any other appropriate materialinstead of silicon dioxide described above. For example, the first layer2 may comprise a silicon nitride (Si₃ N₄) film formed by thermalnitriding, specifically, by annealing the silicon substrate 1 In anammonia (NH₃) atmosphere. Alternatively, the silicon nitride film may beformed by CVD or sputtering.

Then, on the first layer 2, a second layer 3 is deposited by sputteringor vapor deposition.

The second layer 3 is formed of a material having a sufficiently lowetch rate (in other words, having a sufficiently high selection ratio)in dry etching by the use of a reactive gas such as C_(x) F_(y) andC_(x) Cl_(y) in the forms of tons, radicals, or neutral molecules, ascompared with an optical waveguide layer which will later be described.Specifically. the second layer 3 may comprise a metal film formed of ametal material, such as gold (Au), aluminum (A1), tungsten (W), tungstensillcide (WSi), or chromium (Cr), and having a thickness on the order ofseveral hundreds nanoineters to one micrometer.

As Illustrated in FIG. 2A, the second layer 3 is patterned into apatterned second layer 3a in the form of the groove-sculpturing maskpattern. For example, patterning is carried out by a combination ofphotolithography and wet etching using chemicals or ECR (ElectronCyclotron Resonance) plasma etching using argon (Ar) ion plasma.

Inasmuch as the second layer 3 is formed of the above-mentioned metallicmaterial, the second layer 3 can be used also as an electric wiringlayer adapted to flow high-speed signals. Temporarily referring to FIG.3, the second layer 3 is separately patterned into a groove-sculpturingmask pattern 8a and an electric wiring pattern 11 of a desired shape.Thus, the electric wiring layer insulated from the silicon substrate 1can be formed.

Turning to FIG. 2B, an optical waveguide layer 4 is formed on the firstlayer 2 and the patterned second layer 3a. The optical waveguide layer 4is made of, for example, silica glass containing P, Ge, B, or the likeand comprises a waveguide core layer 4a and a waveguide cladding layer4b. The waveguide core layer 4a and the waveguide cladding layer 4b willbe referred to as a core portion and a surrounding portion,respectively.

The optical waveguide layer 4 is formed by a combination of deposition(such as CVD or flame deposition) of the waveguide core layer 4a and thewaveguide cladding layer 4b and etching (such as dry etching by RIE(Reactive Ion Etching))of the waveguide core layer 4a.

As illustrated in FIG. 2B, the optical waveguide layer 4 typically has astructure such that the waveguide core layer 4a is embedded in thewaveguide cladding layer 4b composed of a lower cladding layer 4c and anupper cladding layer 4d.

Preferably, the optical waveguide layer 4 has a size which is givenbelow. The lower cladding layer 4c has a thickness between 10 and 20micrometers. The waveguide core layer 4a has a square section of about5×5 micrometers. The upper cladding layer 4d has a thickness on theorder of 10 micrometers.

The waveguide core layer 4a has a refractive index slightly higher thanthat of the lower and the upper cladding layers 4c and 4d so that thedifference Δn in refractive index is on the order of 5% for example. Inthis case, the optical waveguide layer 4 is of a single-mode type.

In the method according to this invention, the optical waveguide layer 4is not restricted to the single-mode type but may be a multi-mode type.As a material of the optical waveguide layer 4, not only silica glassbut also PMMA or fluorine polyimide can be used.

Turning to FIG. 2C, an end plane forming mask 5 is formed on the opticalwaveguide layer 4 In the following manner. A metal film such as Cr andTi is deposited to a thickness between several tens and several hundredsnanometers. On the metal film, a photoresist is deposited to a thicknessbetween several micrometers and several tens micrometers. Then,photolithography Is carried out to leave the end plane forming mask 5 inthe form of a desired mask pattern. As illustrated in FIG. 2C, the endface forming mask 5 covers an upper surface of the optical waveguidelayer 4 in an area desired to be left while a remaining area is leftuncovered. The remaining area corresponds to a position where a V-shapedgroove or a groove 9 (FIG. 2E) is to be formed in the silicon substrate1 as will later be described.

Thereafter, the optical waveguide layer 4 and the first layer 2 areetched in the remaining area which Is not covered by the end planeforming mask 5. The etching operation is carried out until a substratesurface 7 is exposed. At this time, the optical waveguide layer 4 has awaveguide end plane 6, as illustrated in FIG. 2D.

By way of example, consideration will be made as regards the opticalwaveguide layer 4 of silica glass. In this case, the above-mentionedetching operation is generally carried out by dry etching using ions,radicals, neutral molecules of the reactive gas such as C_(x) F_(y) orC_(x) Cl_(y) so as to suppress side etching. During the etchingoperation, the patterned second layer 3a in the form of thegroove-sculpturing mask pattern depicted at 8 in the figure is notsubstantially etched and remains as a mask in dry etching. This isbecause the patterned second layer 3a is formed of the material having asufficiently low etch rate (in other words, having a high selectionratio) in dry etching using the reactive gas, as compared with theoptical waveguide layer 4.

As a consequence, the first layer 2 under the patterned second layer 3ais patterned Into a patterned first layer 2a in the form of thegroove-sculpturing mask pattern 8, as illustrated in FIG. 2D.

Preferably, the patterned first layer 2a is formed of the silica glassmaterial containing silicon dioxide (SiO₂) as a main component.Therefore, for the groove-sculpturing etchant (for example, the KOHsolution) used in etching the silicon substrate 1, the patterned firstlayer 2a has an etch rate considerably smaller than that of the siliconsubstrate 1. The etch rate of the patterned first layer 2a is variablein dependence upon a film forming process (for example, thermaloxidation of silicon, CVD, or sputtering) and an Impurity concentrationof the silicon dioxide film as the first layer 2. For example, when thefirst layer 2 of silicon dioxide is formed by thermal oxidation and theKOH solution is used as the groove-sculpturing etchant, the patternedfirst layer 2a has an etch rate between one to several hundreds and oneto one thousand of that of the silicon substrate 1. Accordingly, thepatterned first layer 2a can be used as a mask in etching the siliconsubstrate 1.

In the above-mentioned dry etching, side etching can be suppressed bythe use of RIE excellent in vertical linearity. In this event, thegroove-sculpturing mask pattern 8 is exactly and accurately reproducedIn the patterned first layer 2a.

As a result, the groove-sculpturing mask pattern 8 as the patternedfirst layer 2a is formed in a groove-sculpturing area 10 while thesubstrate surface 7 is exposed in a portion without the mask pattern 8,as illustrated In FIG. 2D.

Herein, it is possible to replace a part of the above-mentioned dryetching by wet etching.

Specifically, consideration will be made about etching of an opticalwaveguide of silica glass by way of example. Temporarily referring toFIG. 4, the optical waveguide layer 4 is etched to a certain depth bywet etching using buffered fluoric acid (mixed solution of ammoniumfluoride and hydrogen fluoride). It is noted here that the "certaindepth" is a depth such that the first layer 2 is not exposed. Typically,the optical waveguide layer 4 is etched to leave a thickness betweenseveral tens nanometers (several hundreds angstroms) and 1 micrometer,taking the thickness of the optical waveguide layer 4 and thefluctuation in etching rate into consideration. The wet etching isfollowed by the dry etching. As a consequence, the groove-sculpturingmask pattern 8 as the patterned first layer 2a can be formed with a highaccuracy.

Turning to FIG. 2E, the substrate 1 is etched with the patterned firstlayer 2a used as a mask to form the groove 9 in the substrate 1.

For example, the groove 9 is formed by anisotropic etching of thesilicon substrate 1 by the use of the groove-sculpturing etchant such asthe above-mentioned KOH solution, a NaOH solution, a CsOB solution, amixed solution of ethylenediamine and pyrocatechol, or hydrazine.

Finally, an optical fiber is mounted with the groove 9 used as a guidegroove.

Even if the second layer 3 is formed of a material inherently having nocorrosion resistance to the groove-sculpturing etchant or a materiallosing the corrosion resistance to the groove-sculpturing etchant underthe influence of the process of forming the optical waveguide layer overthe second layer 3, it is possible according to the above-mentionedmethod to accurately and reliably form the groove-sculpturing maskpattern a comprising the patterned first layer 2a without beinginfluenced by the process of forming the optical waveguide layer.

Experimentally, the first layer 2 of the silicon dioxide film was formedon the silicon substrate 1 by thermal oxidation. Then, the second layer3 of a tungsten silicide film was deposited on the first layer 2 andpatterned into the patterned second layer 3a in the form of a groovesculpturing mask pattern. Over the first and the second layers 2 and 3,the optical waveguide layer 4 of silica glass was formed.

Then, dry etching was carried out by RIE using buffered fluoric acid toremove the optical waveguide layer 4 and the first layer 2 in a desiredarea as a V-shaped groove sculpturing area. As a consequence, the firstlayer 2 was patterned Into the patterned first layer 2a in the form ofthe V-shaped groove sculpturing mask pattern with an accuracy on theorder of submicron which is substantially equivalent to that achieved inphotolithography.

Thereafter, the silicon substrate 1 was subjected to anisotropic etchingby the use of the etchant comprising the KOH solution and isopropylalcohol added thereto. The patterned first layer 2a of the silicondioxide film acted as a high-accuracy mask without any substantialdeformation by the immersion in the etchant. As a result, a desiredgroove was obtained at the yield of about 95% or more.

In this state, however, the optical fiber could not be mounted in aproper position. Specifically, as a result of the anisotropic etching,the silicon substrate 1 has a (111) plane 14 (FIG. 2E and 4) adjacent tothe end plane 6 of the optical waveguide layer 4. The (111) plane 14would collide with a fiber end of the optical fiber to inhibit the fiberend from complete contact with the end plane 6 of the optical waveguidelayer 4.

In view of the above. a cut groove 15 was formed between a groovesculpturing area 10a and an optical waveguide area 13 by the use of adicing saw, as illustrated in FIG. 5. The cut groove 15 had a widthbetween 100 and 200 micrometers and a depth greater than that of aV-shaped groove 9a. Thus, the (111) plane 14 which would collide withthe fiber end (not shown) Is removed. In the figure, a hatched portion15a represents a sectional shape of a portion removed by the dicing saw.

Thereafter, the optical fiber was mounted in the groove 9a. Thus, theoptical fiber was mounted in proper alignment with the optical waveguidelayer of silica glass without requiring any special adjustment.

According to the method described above, the guide groove for mountingthe optical fiber can be reliably and accurately formed in the substratewithout being influenced by the process of forming the optical waveguidelayer. Therefore, the optical fiber can be accurately mounted on thesubstrate having the optical waveguide layer without requiring anyspecial adjustment. This enables mass-production which results inreduction of the production cost- Furthermore, the metal materialexcellent in electric characteristic such as a high-frequencycharacteristic can be used as the second layer. Therefore, the electricwiring pattern insulated from the silicon substrate can besimultaneously formed from the second layer on the same substrate. Thus,it is possible to further reduce the production cost.

While this invention has thus far been described in conjunction with thepreferred embodiment, it will readily be understood for those skilled inthe art to put this invention into practice in various other manners. Inthe foregoing description, the groove for mounting the optical fiber isformed on the same substrate on which the optical waveguide layer isformed. However, this invention Is not restricted to the above-mentionedinstance.

For example, this invention is also applicable to the case where twosubstrates are separately prepared one of which is provided with anoptical waveguide layer and the other of which is provided with a grooveas a guide groove for guiding the optical fiber. As a particular case,these substrates are coupled by the use of a coupling groove formed inat least one of the substrates. In the particular case, the couplinggroove can be formed according to the method of this invention.

In the foregoing, description has been made assuming the case where theV-shaped groove is formed on the (100) silicon substrate by theanisotropic etching. However, a different type of the silicon substrate,for example, a (110) silicon substrate may be used in this invention asfar as any groove configuration Is formed in the substrate by etchingWhen the (110) silicon substrate is used, the groove does not have a Vshape. Even in this event, a similar groove-sculpturing mask pattern canbe formed with a high reliability and a high accuracy according to themethod of this invention.

Furthermore, the substrate may be formed of a material other thansilicon. This invention Is also applicable to an indium phosphide (InP)crystal substrate or a gallium arsenide (GaAS) crystal substrate if anetchant is appropriately selected.

What is claimed is:
 1. A method of manufacturing an optical devicehaving a groove, comprising the steps of:forming, on a substrate, afirst layer of a first resist comprising silicon nitride to serve tomask groove-sculpting etching; forming, on a part of said first layer, asecond layer of a second resist, said second resist comprising an upperplane, and said second resist being selected to be resistant to dryetching, and said second layer being patterned to define a groove areaand shape; carrying out a first etching of said first layer locatedwithin said groove area to expose an area of said substrate; carryingout a second etching of said area of said substrate located within saidgroove area thereby to form a groove for accepting an optical fiber; andsubsequent to said forming a second layer, forming an optical waveguidelayer at a plane above said upper plane.
 2. The method of claim 1,wherein said second resist comprises a material selected from the groupconsisting of chromium, tungsten, tungsten silicide, and gold.
 3. Themethod of claim 1, wherein in said step of forming a second layer of asecond resist, said second resist is further selected as an electricwiring material, and wherein said second layer is further patterned todefine an electric wiring.
 4. The method of claim 1, wherein said stepof forming an optical waveguide layer at a plane above said upper planecomprises the further step of forming the waveguide layer on a lowercladding layer having a thickness between 10 to 20 micrometers andoverlying said first and second layers.